1. Field of the Invention
The present invention is directed to a method for eliminating residual oxygen contaminations from crucible-drawn silicon wafers.
2. Description of the Prior Art
The employment of zone-drawn silicon for manufacturing active components is well-known. It has been shown for active power semiconductor components in vertical format, however, that the employment of zone-drawn silicon has limits in mass production, since wafers only having a diameter.ltoreq.150 mm are made.
It is also known to employ wafers known as silicon epitaxy wafers. Such silicon epitaxy wafers are highly doped silicon wafers on which a lightly doped epitaxial silicon layer is applied. If such wafers are to be used in a power semiconductor component, the thickness of this epitaxial layer must be increased as the blocking voltage increases for which the power semiconductor component is designed.
Further, wafers known as crucible-drawn silicon wafers are known, i.e. silicon wafers drawn according to the Czochralsky method. The employment of these crucible-drawn wafers would be economically desirable since wafers having extremely large diameters can be manufactured with this method. Such wafers, however, were hitherto not capable of being utilized in many applications, particularly given vertical power semiconductor components, since crucible-drawn silicon wafers exhibit doping fluctuations ("striations") and imperfections due to incorporated carbon and oxygen contaminants that degrade the component properties.
Crystal-drying from the melt according to Czochralsky is a well-known method for manufacturing single-crystals. Relatively large single-crystals can be manufactured with a suitably oriented inoculation crystal that is brought briefly into contact with the melting surface and is then slowly in turn drawn upwardly, i.e. slower than 1 mm/min. A rotational movement of the inoculation crystal, for example 20 revolutions per minute, insures a uniform crystallization and likewise causes a uniform incorporation of dopants added to the melt.
The temperature profile at the boundary between the melt and the solid crystal is important for growing the crystal in a mechanically stress-free manner as well as for the homogeneity of a doping perpendicular to the direction. Given a non-planar surface of constant temperature, ring structures ("striations") having microscopic doping fluctuations occur, which are extremely disturbing particularly for employment in vertical power semiconductor components.
The selection of the crucible material is critical in the case of silicon. Quartz and graphite, graphite provided with a hard graphite surface layer (shiny coal) as well as boron nitride are suitable.
The high melting temperature of 1415.degree. C. means that contaminates from the crucible material enter into the melt.
The two principal residual contaminants of crucible-drawn silicon single-crystals are slight quantities of oxygen and carbon (approximately 0.02 ppm). The occurring carbon contaminants that arise from the crucible material are usually not critical since the carbon does not have a doping effect in silicon. The oxygen contaminants, however, are problematical.
The oxygen contaminants in crucible-drawn silicon have long been utilized for "intrinsic" gettering. The silicon wafers are for this purpose subjected to a tempering cycle in order to generate an imperfection-free, surface-proximate zone. This tempering cycle is composed of a first high-temperature step at approximately 1100.degree. C. followed by a low-temperature step at approximately 650.degree. C. and a second high-temperature step at approximately 1000.degree. C.
This tempering cycle, which is also called "denuding process" is thereby highly dependent on the initial oxygen and carbon concentration in the silicon.
The first high-temperature step dissolves the existing oxygen dispersions and thus enables the drive-out (expulsion) of the oxygen from the surfaces of the silicon wafer. In the following, second low-temperature step, nucleation sites are generated in the volume of the silicon wafer, i.e., thus under the "denuded zone". During the subsequent high-temperature step dispersions that serve as getter centers for oxygen, heavy metals and other imperfections during the manufacturing process grow at these nucleation sites.
The usable active zone, referred to as the "denuded zone" in this method, is only a few micrometers deep, so that this is not suitable for the employment of silicon wafer manufacture treated in this way for active, vertical power semiconductor components whose space charge zones extend approximately 100 micrometers or even deeper into the volume of the silicon wafer.